Turtles Living in Cramped Quarters

Hi, Last year, my mom bought my little sister two red-earred sliders from a pet store. They told her that they could live happily in a 10 gallon tank and would not get more than a couple of inches across. Well, now she has two big turtles that seem to just keep getting bigger. Now another pet store has told her that sliders can live in her backyard (it's 30 degrees at night where she lives and will get up to the 100s in the summer time.) I told her this probably wasn't a good idea. She is concerned because she is reluctant to buy a bigger tank (i.e. 30 or 40 gallons) because she doesn't know how to care for the tank (oh yeah, the pet store also told her that she didn't need to clean the cage, just needed a filter and that the turtles didn't need a light.)

So, does she need a bigger tank? What size is a good size for 2 turtles? How do you keep a tank that size clean? Do you have to change the water often? Will a filter suffice? I figure that if I don't get figure this out, they will continue to live in cramped quarters.